Microstructural evolution of CdTe-based solar cells during chlorine activation

太阳能电池市场目前由第一代晶圆硅基太阳能电池主导（>80%）。硅是光的不良吸收体，因此需要相对大量的材料。直接带隙半导体（诸如CdTe）具有高得多的光吸收效率，使得可以制造薄膜太阳能电池。然而，为了使基于CdTe的太阳能电池挑战硅晶片，必须提高其整体器件效率。CdTe太阳能电池总是经历氯“活化”处理，其中CdCl 2薄层沉积在CdTe表面上，并在400 ℃的温度下退火20-30分钟。该工艺将器件效率从约1-3%提高到约10- 14%。尽管效率提高了10倍，但由于难以在适当的长度尺度上表征微观结构（主要机制被认为是由于氯偏析导致的晶界钝化），对活化过程仍然知之甚少。在这个项目中，氯活化过程中发生的微观结构变化的特点是使用电子显微镜技术。PI开发了一种新的，基于阴极发光的方法，用于确定在真实的器件结构中单个晶界的复合速度。因此，现在可以首次研究晶界对太阳能电池效率的作用。在过去几年中，仪器仪表也取得了重要进展。特别地，单色电子显微镜使得能够以仅几纳米的空间分辨率测量局部光学性质（例如，带隙、吸收系数）。在氯活化期间，硫相互扩散发生在p-n结（即CdS-CdTe界面）处，这影响照射期间的载流子生成。伦敦帝国理工学院的单色电子显微镜将被用于验证硫相互扩散对p-n结光学特性的影响，并了解这是如何影响器件效率的。氯化镉的蒸发温度低，是水溶性的，因此大规模处理（例如在工业规模制造中）是危险的。因此，还将探索替代的、更安全的活化方法，例如使用含氯气体。使用含氯气体激活的太阳能电池的微观结构将与CdCl 2激活的太阳能电池进行比较，并与测得的效率增加相关。实验结果将被纳入一个模拟太阳能电池操作的计算机程序。该计划的目的是确定支持氯活化的主要机制，以及快速筛选旨在优化太阳能电池效率的潜在加工路线。后者是太阳能电池制造方法的范式转变，远离基于试错的方法，这些方法耗时且昂贵。

The solar cell market is currently dominated (>80%) by first generation, wafer silicon-based solar cells. Silicon is a poor absorber of light and consequently relatively large volumes of material are required. A direct band gap semiconductor, such as CdTe, has a much higher efficiency for light absorption, so that thin film solar cells can be fabricated. However, in order for CdTe-based solar cells to challenge wafer silicon, its overall device efficiency must be improved. CdTe solar cells always undergo a chlorine 'activation' treatment, where a thin layer of CdCl2 is deposited on the CdTe surface and annealed at a temperature of 400OC for 20-30 mins. This process increases the device efficiency from ~1-3% to ~10-14%. Despite the 10-fold increase in efficiency the activation process remains poorly understood, due to the difficulty in characterising the microstructure at the appropriate length scales (the dominant mechanism is thought to be passivation of grain boundaries due to chlorine segregation). In this project microstructural changes taking place during chlorine activation are characterised using electron microscopy techniques. The PI has developed a novel, cathodoluminescence based method for determining the recombination velocity of an individual grain boundary in a real device structure. Hence it is now possible to examine the role of grain boundaries on solar cell efficiency for the first time. There have also been important advances in instrumentation over the last few years. In particular, monochromated electron microscopes enable local optical property (e.g. band gap, absorption coefficient) measurement at spatial resolutions of only a few nanometres. During chlorine activation, sulphur inter-diffusion takes place at the p-n junction (i.e. the CdS-CdTe interface), which affects carrier generation during illumination. The monochromated electron microscope at Imperial College London will be used to characterise the effects of sulphur inter-diffusion on optical properties of the p-n junction, and understand how this affects device efficiency.CdCl2 has a low evaporation temperature and is water soluble, making it hazardous to handle on a large scale (e.g. in industrial-scale manufacture). Hence alternative, safer methods for activation, such as the use of chlorine containing gases, will also be explored. The microstructure of solar cells activated using chlorine containing gases will be compared to CdCl2 activated solar cells, and correlated with the measured increase in efficiency. Experimental results will be incorporated into a computer programme for modelling solar cell operation. The purpose of the programme is to identify the dominant mechanism(s) underpinning chlorine activation as well as rapid screening of potential processing routes designed to optimise solar cell efficiency. The latter is a paradigm shift in solar cell fabrication methodology, moving away from methods based on trial and error, which are time consuming and costly.